The invention relates to sensor systems and, more particularly, to non-imaging scanning sensor systems.
Non-imaging scanning systems are employed in many applications where it is desired to detect the presence of objects of interest. For example, non-imaging scanning systems employing an array of infrared detector elements positioned in the focal plane of a scanning optical system are used to passively detect the presence of vehicles at extended distances. Such systems initially employed a single column of detector elements arranged in a line array at the focal plane of an optics system. The line array is mounted in a gimballed sensor unit to scan a portion of a field of view and produce detector output signals which are sampled and multiplexed for further processing by on-gimbal circuitry. An improvement to basic line array systems, known as Time Delay Integration (TDI), employs a plurality of detector elements adjacently positioned in the focal plane in the scan direction. Each detector element is sampled in-phase and the resultant sampled signal is applied to a delay circuit having a delay value representative of the position of the connected detector element in the scan direction. The outputs of the delay circuits are superimposed in time and summed to provide an output signal from the TDI array having an improved signal to noise ratio compared to line arrays.
It is well known that the performance of TDI systems can be improved by increasing the number of detector elements mounted in the focal plane of the optical system. Advances in semiconductor technology now provide arrays of hundreds or even thousands of infrared detector elements, and future advances providing even larger arrays of detector elements will provide further increases in system performance. Such large numbers of detector elements in the past, however, have called for a very high sampling rate of the detector elements. This has resulted in a corresponding increase in the complexity of processing circuitry. An even more limiting factor is the large number of interconnections which are required to carry output signals from large detector element arrays to circuitry mounted within the sensor unit, known as on-gimbal circuitry. Moreover, prior art sensor methods and apparatus employing large TDI detector element arrays have required correspondingly large delay circuits. This also increases the size, complexity, and weight of apparatus mounted in the sensor unit.
Various techniques are known for improving the performance of the basic TDI system. For example, it is well known to provide a second array of detector elements offset in the cross-scan direction by an amount equal to one-half the cross-scan dimension of each detector element. This provides a spatial sample rate which is double the sample rate in the cross scan direction obtainable with only a single array of detector elements.
Another method of increasing the performance of prior art TDI systems involves the introduction of a progressively increasing phase shift in the relative timing of the sampling signals and initial contact of the image at successive detector elements. The composite output signal of the array then corresponds to an average over all possible relative timings of sampling signals and initial contact of the image at a detector element. This method is shown, for example, in U.S. Pat. No. 4,327,377 to Takken issued Apr. 27, 1982.
Smaller delay lines, lower complexity circuitry, and fewer connections between the focal plane and on gimbal circuitry could be provided by lowering the detector element sample rate. However, reducing the effective sample rate of the system results in a degradation in system performance, since the Nyquist theorem specifies that a waveform having a given frequency, such as a detector output signal produced by a target object, must be sampled at a rate no less than twice that frequency in order to faithfully reproduce the waveform. Thus, reduction of the effective sample rate of the output signal of the scanning system would result in a loss of ability to detect and accurately track small amplitude targets. None of the methods and apparatus involving time delay integration of the prior art are thus completely satisfactory in reducing the circuit complexity and interconnection requirements of scanning systems, while maintaining the desired level of sensor performance.